An electrical power system comprises transmission and distribution lines and electric power plants situated in discrete locations. Power is generated at a power plant and sent through high voltage transmission lines to substations located some distance away. The voltage from the power plant is stepped up through a step-up transformer to a high voltage transmission line which transmits the power to a substation. The substation includes a step-down transformer which steps the voltage down to low voltage distribution lines which feed power to residential and commercial customers. This invention relates to protection of the low voltage distribution lines which feed power to residential and commercial customers.
There are numerous devices for protection of high voltage transmission lines (power plant to substation lines) comprising either electromechanical or electronic relay systems which monitor voltages and currents for the purpose of detecting faults and operating conditions in the various high voltage transmission lines. These transmission lines are protected against fault conditions in order to avoid serious equipment damage and personal injuries. Such a fault condition, for instance, might occur when transmission line insulator failure causes the transmission line to fall from an elevated position and to make contact with the ground. Line currents under such a condition can increase to several times normal current and can destroy or damage both lines and the attached equipment if the faulted line section is not cleared within a very short time after the fault condition occurs.
The prior art does not provide an adequate protective device to guard against dangers which can result from similar fault conditions on low voltage distribution lines, i.e., lines from substations to customers. Prior devices such as one disclosed in U.S. Pat. No. 3,327,170 detect broken conductors on distribution lines by generating harmonics on unbroken phase lines of the same distribution circuit. However, these devices require a three phase wye system and will not detect loss of all three phases. Another prior art device, disclosed in U.S. Pat. No. 3,840,783, detects grounds on ungrounded polyphase power distribution systems but can only be used on a three phase delta system and cannot be used on a four-wire wye system.
Other prior art devices must compare at least three phases in order to detect a phase loss in any one of the phases. Still other devices require a balanced load in all three phases for determining a loss of voltage in any one phase and are limited in sensitivity by the amount of imbalance. More commonly, power distribution lines are protected by overcurrent devices which are tripped by high current values to interrupt power to the line. Unfortunately, fallen lines do not always draw sufficient current to trip such devices thus creating unsafe conditions for persons or animals which may contact such fallen lines.